Move code notifying users of what settings are being used to netlify-config

[Benaiah]

Which problem is this feature request solving?

Currently, the buildbot handles notifying users of what final settings are being used for their build.

Describe the solution you'd like

This logic would be better to have in netlify-config than in the buildbot for a few reasons:

• There's potential for bugs due to handling the config overriding the UI in two places.
• The buildbot cannot distinguish between a UI value that was the same as the value in the config file and a UI value that was used because no value was set in the config file.
• The notifications of what values are being used can only come before or after the warnings output by netlify-config, and cannot be interleaved with them.

Describe alternatives you've considered

We can keep the logic in buildbot.

Additional context

Transferred to netlify/build from https://github.com/netlify/buildbot/issues/1413. This issue was an outgrowth of the discussion on https://github.com/netlify/buildbot/pull/1380.